Dioxygen (O2)-derived oxidizing species are major participants in the initiation and propagation of many mammalian diseases. The molecular role of oxidants in disease centers around functional impairment of cell membranes through the initiation of lipid peroxidation, thus a rational approach towards preventative therapeutic intervention can be based on controlling this initiation process. Unfortunately, the initiation process is the least mechanistically understood aspect of lipid peroxidation. This proposal is designed to provide a detailed mechanistic description of lipid peroxidation initiation in model membranes (liposomes) as an essential foundation for the future study of this process in vivo. The working hypothesis is that the mechanism of lipid peroxidation initiation is both oxidant- and environment-dependent. Specific experimental goals are as follows. First, the biologically significant oxidants will be evaluated as to their ability to initiate lipid peroxidation in chemically defined liposomes containing the necessary structural elements for oxidant susceptibility. This evaluation will define both chemical and environmental (i.e.-location of generation) requirements for the different oxidants. Second, a parallel evaluation will be preformed with liposomes containing biologically appropriate concentrations of lipid hydroperoxides, to evaluate lipid peroxidation sensitization and differential oxidant effects due to the presence of these reactive species. Third, the mechanisms of initiation will be determined for all oxidants active in inducing lipid peroxidation in both unperoxidized and peroxidized liposomes. A comparison between mechanisms of initiation of the different oxidants should allow for the proposal of metabolite assays applicable in vivo for the sources and identities of biologically active oxidants. Preliminary results have defined a central role for the perhydroxyl radical, the conjugate acid of the most biologically ubiquitous oxygen species, superoxide, in lipid peroxidation initiation. In addition, fatty acid hydroperoxides sensitize lipids to perhydroxyl radical-dependent lipid peroxidation. The preliminary results enable a postulation of the synergistic role of the perhydroxyl radical and lipid hydroperoxides in heart disease, stomach disease and the mechanism of action of various drugs. Thus, it is anticipated that the results of these studies will open various avenues for the design and implementation of therapeutic interventions of oxidant-induced disease states-the long-term goal of this research program.